1. Field of the Invention
This invention relates to lifting assemblies. More particularly, the invention relates to lifting assemblies of the type which include a load beam, one or more cross arms suspended from the load beam, and lifting devices attached to the cross arm or arms; typical of such assemblies are those including lifting pads which adhere by vacuum to flat sheets of metal or the like to lift them without causing surface or edge damage.
2. Description of Related Art
In using lifting assemblies of the type described, it is frequently necessary to move a cross arm from one position to another along the longitudinal axis of the load beam--e.g., to accommodate sheets of different sizes--and/or to position a cross arm at an angle other than 90.degree. with respect to the load beam--e.g., to accommodate sheets of various shapes.
A typical load beam for a lifting assembly of the type described is hollow, of generally square or rectangular cross section, and has a slot extending along the longitudinal center line of the bottom wall. Typically, cross arms of prior art assemblies are suspended from the load beam by means including a metal plate or suspending pad positioned inside the load beam and bridging the slot, a downwardly extending member fastened to the lower pad surface and projecting through the slot, and means securing the cross arm to the downwardly extending member. Additionally, means may be provided to lock the pad in a desired longitudinal and/or angular position along the load beam.
Several shortcomings exist with the above-discussed prior art lifting assemblies. Chief among these is the difficulty of moving the cross arm from one position to another along the load beam, due to the fact that the weight of the cross arm and any lifting devices attached thereto causes high friction between the suspending pad and the load beam bottom wall. Additionally, the means for securing the cross arm to the above-mentioned downwardly extending member and locking it in angular and longitudinal position typically comprise merely a lock nut threaded on to the lower end of the downwardly extending member below the cross arm. Under the full weight of the cross arm it is frequently very difficult to re-tighten the lock nut after loosening it to change the position or the angle of the cross arm; conversely, however, vibration and impact during use of the lifting device can cause the lock nut to become loose and, if not noticed, come completely off, with the expected consequence that the cross arm falls from the load beam.